1. Field of the Invention
The present invention relates to an image display device, such as a CRT display device, which is used, for example, in an image file apparatus.
2. Description of the Related Art
In a conventional CRT display device which has been used in an image file apparatus, a display image screen corresponds to a memory, unlike an ordinary CRT of such a type as to enable an electron beam to scan the screen from edge to edge. Such a display image device does not perform an edge-to-edge scanning for economy.
The relation of a display raster to the fluorescent screen of, for example, a CRT display device is as shown in FIG. 1 where a represents the outermost peripheral edge of the fluorescent screen A of the CRT and b the outermost periphery of the display raster B. Generally, the corner A' of the fluorescent screen A in FIG. 1 is curved and the display raster B is rectangularly so shaped that an effective display area is provided on the fluorescent screen. The distance between the outermost periphery b of the display raster B and the outermost peripheral edge a of the fluorescent screen A is the smallest at each corner of the fluorescent screen in FIG. 2.
In this type of CRT display device, a display position varies due to the effect of the geomagnetism and ambient temperature for example. If a smaller clearance is provided between the outermost peripheral edge a of the fluorescent screen A and the outermost periphery b of the display raster B, the rectangular display area of the display raster B projects from the fluorescent screen A as shown in FIG. 3. It is, therefore, necessary to decrease a display area of the display raster B or to increase a CRT size. Even in a CRT whose display position, etc., undergo only a small enough variation, it is not possible to narrow the distance between the horizontal and vertical lines at the outermost peripheral edge of the fluorescent screen A and those lines at the outermost periphery of the display raster B, due to the corner of the fluorescent screen A curved, despite the fact that it is desired to take as large a display surface as possible on the display raster.
Since the rectangular display area is provided on the display raster B, an unused display area is left at the peripheral edge of the fluorescent screen and, in this sense, the phosphor screen cannot be used effectively.